The time course of postnatal development of cardiovascular regulatory mechanisms will be studied in piglets of differing age, from the viewpoint of efferent outflow of the regulatory system. Animals will be lightly anesthetized with halothane or decerebrated. Cardiovascular responses will include changes in heart rate, blood pressure, and renal and mesenteric flows (electromagnetic transducers). Spontaneous and evoked sympathetic activity will be sampled by recording from the splanchnic and renal nerves and analyzed by averaging computation and auto- and cross-correlation techniques. The cardiovascular responses to stimulation of atrial mechanoreceptors will be determined and their role in overall cardiovascular regulation evaluated. The effects of stresses such as hypercapnia and hemorrhage on cardiovascular regulation will also be evaluated. Age-dependency of results will be determined statistically in a population of piglets and compared with mature miniature swine. The results should be relevant to our understanding of early circulatory extrauterine adaptation and to our knowledge of the degree of regulation of cardiovascular function available in the neonate. Information will be obtained which will be pertinent to the understanding of reflexes involved in physiological adjustments to various stresses (hypercapnia, hemorrhage).